Lecture on the First Law of Thermodynamics

Introduction

• Presenter: Professor Dave
• Topic: First law of thermodynamics, also known as the law of energy conservation.

Key Concepts

• First Law of Thermodynamics Equation:
  • [ \Delta U = Q - W ]
    • ( \Delta U ): Change in internal energy of the system (measured in joules)
    • ( Q ): Heat transferred to/from the system (measured in joules)
    • ( W ): Work done by/on the system (measured in joules)

Types of Processes

1. Isovolumetric Process (Constant Volume):

   • Work ( W = 0 ) since no pressure-volume work can be done.
   • ( \Delta U = Q )
   • Example: Bomb calorimeter.

2. Isothermal Process (Constant Temperature):

   • ( \Delta U = 0 )
   • ( Q = W )
   • Example: Ideal car engine converting heat energy into work.

3. Adiabatic Process (No Heat Transfer):

   • ( Q = 0 )
   • ( \Delta U = -W )
   • Example: Atmospheric processes where air changes position due to pressure.

4. Isolated System:

   • No heat transfer and no work done.
   • ( \Delta U = 0 ).

Sign Conventions

• Heat ( Q ):
  • Positive when absorbed by the system
  • Negative when lost by the system.
• Work ( W ):
  • Positive when done by the system (e.g., expanding gas)
  • Negative when done on the system (e.g., gas compression).

Calculation Example

• Problem: 100 joules of compression work is done on a system and internal energy increases by 74 joules.
• Calculate: Energy transferred as heat.
  • Rearrange the equation: [ Q = \Delta U + W ]
  • Substitute values: ( \Delta U = 74 ) joules, ( W = -100 ) joules.
  • Result: ( Q = -26 ) joules (heat lost by the system).

Conclusion

• Frequent use of the first law equation in calculations.
• Importance of correct sign usage in calculations.

Additional Notes

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